251 Design

Stirling engines operate on a closed thermodynamic cycle where a temperature differential is converted into mechanical and/or electrical power. External heat is supplied at a high temperature to the engine heater head, and thermodynamic waste heat is rejected to ambient temperature. An internal displacer piston physically shuttles the helium working fluid between the hot and cold regions, creating a varying pressure value. That pressure wave causes the power piston to reciprocate. The reciprocating motion can be used to produce shaft power similar to an IC engine or may be used to generate electricity directly using a linear alternator. At no time during the cycle does the working fluid enter or leave the engine, which is hermetically sealed. Therefore, the cycle is defined as closed.

FIGURE 2.10

A grid-connected Stirling micro-cogeneration system.

FIGURE 2.10

A grid-connected Stirling micro-cogeneration system.

Several varieties of Stirling engines have been developed by both private and government organizations. The varieties may be grouped into two fundamental categories: kinematic and free-piston. Kinematic engines have a crankshaft and flywheel and may be used in place of internal combustion engines to provide shaft power. The disadvantage of kinematic engines is that the rotating shaft and reciprocating rods must be sealed so that the working fluid does not leave the engine and lubricants do not mix with the working fluid. Due to the difficulty of successfully sealing the shaft, kinematic Stirling engines have limited life and reliability. Free-piston engines, however, have no crankshaft and no seals to maintain. The generator portion, a linear alternator, can be sealed in a pressure vessel along with the engine so that the only items penetrating the pressure vessel are feed-throughs for the electrical output. The two pistons in a free-piston Stirling engine are mounted to allow free axial motion but little or no radial displacement. This is typically accomplished by employing flexural or gas bearings. Other than the mountings, the pistons do not come into contact with any part of the engine, so there are no lubricants needed and no rubbing parts to wear out.

Free-piston Stirling generators can be understood as thermally actuated mass-spring-damper systems. The pistons and moving portion of the alternator provide the masses, their mountings provide the springs, and the magnetic field of the linear alternator provides the damping. Once these values are determined, the engine can be mathematically modeled using linear second order differential equations with known solutions. As with any mass-spring system, a designer may control the natural frequency of the system by altering the mass or spring force. This way, free-piston Stirling generators can be designed to produce AC power at whatever voltage and frequency the application requires. These systems are also load following when attached to the power grid, so if the frequency of the grid changes slightly, the engine will simply change its operating frequency to suit. It is important to note that some load must always be applied to a running free-piston Stirling generator in order to prevent damage to the engine from over-stroke of the pistons. The AC power can be easily converted to DC to charge batteries or operate electronics.

When employed as a remote battery charger (Figure 2.11), a Stirling-powered generator can run continuously and requires only one-quarter the amount of batteries required for a gasoline or diesel system. This configuration could power a remote telecommunications relay, an automated pipeline monitoring station, or an off-grid home.

FIGURE 2.11

Schematic of a battery charger using a free-piston Stirling generator.

FIGURE 2.11

Schematic of a battery charger using a free-piston Stirling generator.

While there are no fundamental limits on power output, the capacity of most free-piston Stirling engines currently available is under 5 kW. This is partly due to thermodynamic and heat transfer considerations as well as the mechanics of mounting the pistons. The heater head, where the heat energy is supplied to the cycle, for small capacity engines may be fabricated from simple monolithic shapes. In larger capacity engines, those with a 7 kW or greater output, such a monolithic heater head does not provide adequate surface area for the required heat energy input. These typically require complex and more costly tubular heat exchangers to get the required heat energy into the cycle for full power operation. The size of the pistons and their amplitude serve to further complicate the design of larger capacity engines since they require more complex mounting technology. These requirements could increase the cost and size of larger engines substantially. The smaller engines (under 5 kW) are easier to design and build, and may be most cost effective.

The efficiency of a Stirling generator system is affected by a number of variables including fuel type, operating temperatures, and mechanical design of the engine. Currently available Stirling engines have generator efficiencies, which are the ratio of engine heat energy input to electrical power output, ranging from nearly 30% for systems as small as 50 W to around 40% for 3 to 5 kW capacity generators. The generator efficiency is largely determined by the efficiency of the alternator and the effectiveness of the regenerator used for the Stirling cycle. The regenerator is the most important and often most expensive single part of the engine. Practical engines must be designed with cost in mind. Therefore, some efficiency is often sacrificed to lower the production cost of the engine.

The total system efficiency (the ratio of fuel input to electric power output) is largely affected by the system employed to supply heat to the engine. In most cases, gaseous fuel such as propane is combusted in a burner, and the resulting heat is transferred to the engine heater head via convection or radiation. Burner technology is constantly improving. Currently, simple, cost-effective burners are 50% efficient. Recuperative burners can achieve much higher efficiencies but at a greater cost. It should be noted that Stirling generator systems are ideally suited for micro-cogeneration, where the exhaust heat from the burner and waste heat from the engine are used for water and space heating. With appropriate heat recovery techniques, cogeneration systems can approach 98% efficiency since nearly all of the heat energy is used in some way.

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